U.S. patent application number 12/390168 was filed with the patent office on 2009-12-03 for printed wiring board.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Tomoyuki ABE, Shin HIRANO, Kenji IIDA, Yasutomo MAEHARA, Hideaki YOSHIMURA.
Application Number | 20090294166 12/390168 |
Document ID | / |
Family ID | 41378371 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090294166 |
Kind Code |
A1 |
YOSHIMURA; Hideaki ; et
al. |
December 3, 2009 |
PRINTED WIRING BOARD
Abstract
Large-sized through holes are formed in a core layer of a
printed wiring board. Large-sized vias are formed in the shape of a
cylinder along the inward wall surfaces of the large-sized through
holes located within a specific area. A filling material fills the
inner space of the large-sized via. A small-sized through hole
penetrates through the corresponding filling material along the
longitudinal axis of the small-sized through hole. A small-sized
via is formed in the shape of a cylinder along the inward wall
surface of the small-sized through hole. The filling material and
the core layer are uniformly distributed within the specific area
in the in-plane direction of the core substrate. This results in
suppression of uneven distribution of thermal stress in the core
layer in the in-plane direction of the core layer.
Inventors: |
YOSHIMURA; Hideaki;
(Kawasaki, JP) ; IIDA; Kenji; (Kawasaki, JP)
; ABE; Tomoyuki; (Kawasaki, JP) ; MAEHARA;
Yasutomo; (Kawasaki, JP) ; HIRANO; Shin;
(Kawasaki, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
41378371 |
Appl. No.: |
12/390168 |
Filed: |
February 20, 2009 |
Current U.S.
Class: |
174/264 |
Current CPC
Class: |
H05K 2201/09809
20130101; H05K 2201/0959 20130101; H05K 3/4608 20130101; H05K
2201/09609 20130101; H05K 3/445 20130101; H05K 2201/09536 20130101;
H05K 2201/096 20130101; H05K 2201/0323 20130101 |
Class at
Publication: |
174/264 |
International
Class: |
H05K 1/11 20060101
H05K001/11 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2008 |
JP |
2008-143449 |
Claims
1. A printed wiring board comprising: a core layer having
electrical conductivity; large-sized through holes formed in the
core layer, the large-sized through holes penetrating through the
core layer from a front surface of the core layer to a back surface
of the core layer; a large-sized via having electrical
conductivity, the large-sized via formed in a shape of a cylinder
along an inward wall surface of each of all the large-sized through
holes located within a specific area; a filling material filling an
inner space defined in the large-sized via; a small-sized through
hole formed in the filling material, the small-sized through hole
penetrating through the filling material along a longitudinal axis
of the small-sized through hole; and a small-sized via having
electrical conductivity, the small-sized via formed in a shape of a
cylinder along an inward wall surface of the small-sized through
hole.
2. The printed wiring board according to claim 1, wherein the core
layer contains carbon fiber exposed at the inward wall surface of
the large-sized through hole, the carbon fiber connected to the
large-sized via.
3. The printed wiring board according to claim 2, further
comprising an electrically-conductive pattern formed on at least
one of the front and back surfaces of the core layer, the
electrically-conductive pattern connected to the large-sized
via.
4. The printed wiring board according to claim 1, further
comprising a build-up layer formed on at least one of the front and
back surfaces of the core layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No. 2008-143449
filed on May 30, 2008, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The embodiments discussed herein are related to a printed
wiring board including a core layer having electrical
conductivity.
BACKGROUND
[0003] A printed wiring board such as a probe card is well known.
The probe card is utilized for examining a semiconductor wafer and
an LSI (large-scale integrated circuit) chip package, for example.
A semiconductor wafer or an LSI chip package is set on the probe
card. A high temperature operation test such as a burn-in test, or
a low temperature operation such as screening is executed. The
probe card is subjected to a change in the temperature, namely
thermal stress such as a heat cycle. The temperature is changed
within a range set for the high temperature test or the low
temperature test.
[0004] The LSI chip includes a silicon substrate, for example.
Since the coefficient of thermal expansion of silicon is relatively
low, the coefficient of thermal expansion of the LSI chip can be
set relatively low. The core substrate of the probe card is made
out of carbon fiber cloth impregnated with a resin material, for
example. The carbon fiber cloth serves to reduce the coefficient of
thermal expansion of the core substrate. The coefficient of thermal
expansion of the probe card can be equalized to that of the LSI
chip. The electrically-conductive pads of the probe card can thus
be positioned to their respective electrode pins of the LSI chip
with accuracy, for example.
[0005] Secondary through holes are formed in the core substrate of
the probe card. A cylindrical large-sized via having a large
diameter is formed on the inner wall surface of the individual
large-sized through hole. An inner space defined in the large-sized
via is filled with a secondary filling material made of a resin
material. A through hole is formed in the secondary filling
material. A cylindrical small-sized via having a small diameter is
formed on the inner wall surface of the through hole. An inner
space defined in the small-sized via is filled with a filling
material. The filling material is made of epoxy resin, for example.
In this manner, a double via is formed.
[0006] Simultaneously, certain one or ones of the large-sized
through holes have a cylindrical via formed on the inward wall
surface thereof. An inner space defined in the via is filled with a
filling material. No through hole is formed in the filling
material. In this manner, a single via is formed. The carbon fiber
cloth allows the core substrate to be electrically conductive. The
carbon fiber cloth is exposed on the inward wall surfaces of the
large-sized through holes. Since the via is electrically connected
to the core substrate, the core substrate can function as a power
supply layer or a ground layer.
[0007] The core substrate has the single via and the double vias
formed therein. The amount of the carbon fiber cloth and the amount
of the filling material are thus uneven in the in-plane direction
of the core substrate. Since the coefficient of thermal expansion
of the carbon fiber cloth is different from that of the filling
material, distortion is induced in the in-plane direction of the
core substrate in response to thermal stress during a heat cycle
test, for example. The distortion causes a so-called crack. The
crack results in breaking of the electrically-conductive
pattern.
SUMMARY
[0008] According to an aspect of the invention, a printed wiring
board includes: a core layer having electrical conductivity;
large-sized through holes formed in the core layer, the large-sized
through holes penetrating through the core layer from the front
surface of the core layer to the back surface of the core layer; a
large-sized via having electrical conductivity, the large-sized via
formed in the shape of a cylinder along the inward wall surface of
each of all the large-sized through holes located within a specific
area; a filling material filling the inner space defined in the
large-sized via; a small-sized through hole formed in the filling
material, the small-sized through hole penetrating through the
filling material along the longitudinal axis of the small-sized
through hole; and a small-sized via having electrical conductivity,
the small-sized via formed in the shape of a cylinder along the
inward wall surface of the small-sized through hole.
[0009] The object and advantages of the embodiment will be realized
and attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory and are not restrictive
of the embodiment, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an enlarged partial sectional view schematically
illustrating the cross-section of a printed wiring board according
to an embodiment of the present invention;
[0011] FIG. 2 is a sectional view taken along the line 2-2 in FIG.
1;
[0012] FIG. 3 is an enlarged partial sectional view schematically
illustrating a process of overlaying prepregs on one another;
[0013] FIG. 4 is an enlarged partial sectional view schematically
illustrating the process of overlaying prepregs on one another;
[0014] FIG. 5 is an enlarged partial sectional view schematically
illustrating a process of forming large-sized through holes in a
core layer;
[0015] FIG. 6 is an enlarged partial sectional view schematically
illustrating a process of forming a copper plating layer on the
core layer;
[0016] FIG. 7 is an enlarged partial sectional view schematically
illustrating a process of pouring a resin material into the
large-sized through holes;
[0017] FIG. 8 is an enlarged partial sectional view schematically
illustrating a process of overlaying a prepreg and a copper foil on
each of the front and back surfaces of the core layer;
[0018] FIG. 9 is an enlarged partial sectional view schematically
illustrating the process of overlaying a prepreg and a copper foil
on each of the front and back surfaces of the core layer;
[0019] FIG. 10 is an enlarged partial sectional view schematically
illustrating a process of forming through holes in the core
layer;
[0020] FIG. 11 is an enlarged partial sectional view schematically
illustrating a process of forming a copper plating layer;
[0021] FIG. 12 is an enlarged partial sectional view schematically
illustrating a process of forming a copper plating layer;
[0022] FIG. 13 is an enlarged partial sectional view schematically
illustrating a process of forming electrically-conductive
lands;
[0023] FIG. 14 is an enlarged partial sectional view schematically
illustrating a process of forming an insulating layer on the
surface of the core substrate;
[0024] FIG. 15 is an enlarged partial sectional view schematically
illustrating a process of forming a copper plating layer on the
insulating layer;
[0025] FIG. 16 is an enlarged partial sectional view schematically
illustrating a process of forming an electrically-conductive
pattern on the insulating layer;
[0026] FIG. 17 is an enlarged partial sectional view schematically
illustrating the cross-section of a printed wiring board according
to another embodiment of the present invention; and
[0027] FIG. 18 is a sectional view taken along the line 18-18 in
FIG. 17.
DESCRIPTION OF EMBODIMENTS
[0028] Embodiments of the present invention will be explained below
with reference to the accompanying drawings.
[0029] FIG. 1 schematically illustrates the cross-section of a
printed wiring board 11 according to an example of the present
invention. The printed wiring board 11 is utilized as a probe card,
for example. Such a probe card is set in a probe apparatus, for
example. It should be noted that the printed wiring board 11 may be
utilized in any other electronic apparatus.
[0030] The printed wiring board 11 includes a core substrate 12.
The core substrate 12 includes a core layer 13 in the form of a
thin plate. The core layer 13 includes an electrically-conductive
layer 14. Carbon fiber cloth is embedded in the
electrically-conductive layer 14. The fibers of the carbon fiber
cloth extend along the front and back surfaces of the core layer
13. This results in a significant restriction of the thermal
expansion of the electrically-conductive layer 14 in the in-plane
direction. The carbon fiber cloth has an electrical conductivity.
The carbon fiber cloth is impregnated with a resin material so as
to form the electrically-conductive layer 14. The resin material is
a thermosetting resin such as epoxy resin. The carbon fiber cloth
is a woven or nonwoven cloth made of carbon fiber yarns.
[0031] The core layer 13 includes core insulating layers 15, 16
overlaid on the front and back surfaces of the
electrically-conductive layer 14, respectively. The
electrically-conductive layer 14 is sandwiched between the core
insulating layers 15, 16. The core insulating layers 15, 16 are
insulative. Glass fiber cloth is embedded in the core insulating
layers 15, 16. The fibers of the glass fiber cloth extend along the
front and back surfaces of the core layer 13. The glass fiber cloth
is impregnated with a resin material so as to form the core
insulating layers 15, 16. The resin material is a thermosetting
resin such as epoxy resin. The glass fiber cloth is a woven or
nonwoven cloth made of glass fiber yarns.
[0032] Large-sized through holes 17 are formed in the core layer
13. The large-sized through holes 17 penetrate through the core
layer 13 from the front surface to the back surface of the core
layer 13. The large-sized through holes 17 each define a columnar
space. The longitudinal axis of the columnar space is set
perpendicular to the front and back surfaces of the core layer 13.
The individual large-sized through hole 17 defines circular
openings on the front and back surfaces of the core layer 13,
respectively. The carbon fiber cloth of the electrically-conductive
layer 14 is exposed at the inward wall surface of the individual
large-sized through hole 17.
[0033] A large-sized via 18 having a large diameter is formed in
the individual large-sized through hole 17. The large-sized via 18
is electrically conductive. The large-sized via 18 is formed in the
shape of a cylinder along the inward wall surface of the
large-sized through hole 17. Since the carbon fiber cloth is
exposed at the inward wall surface of the large-sized through hole
17 as described above, electrical connection is established between
the large-sized via 18 and the carbon fiber cloth. The large-sized
via 18 is connected to annular electrically-conductive lands 19 on
the front and back surfaces of the core layer 13. The
electrically-conductive lands 19 extend on the front and back
surfaces of the core layer 13. The large-sized via 18 and the
electrically-conductive lands 19 are made of an
electrically-conductive material such as copper.
[0034] The inner space of the large-sized via 18 in the large-sized
through hole 17 is filled with a filling material 21 made of a
resin material. The filling material 21 takes the form of a
cylinder along the inward wall surface of the large-sized via 18.
The filling material 21 is a thermosetting resin such as epoxy
resin. A ceramic filler is embedded in the epoxy resin, for
example.
[0035] The core substrate 12 includes insulating layers 22, 23
overlaid on the front and back surfaces of the core layer 13,
respectively. The back surfaces of the insulating layers 22, 23 are
received on the front and back surfaces of the core layer 13,
respectively. The core layer 13 is sandwiched between the
insulating layers 22, 23. The insulating layers 22, 23 cover over
the exposed surfaces of the filling material 21. The insulating
layers 22, 23 are insulative. Glass fiber cloth is embedded in the
insulating layers 22, 23. The fibers of the glass fiber cloth
extend along the front and back surfaces of the core layer 13. The
glass fiber cloth is impregnated with a resin material for forming
the insulating layers 22, 23. The resin material is a thermosetting
resin such as epoxy resin. The glass fiber cloth is a woven or
nonwoven cloth made of glass fiber yarns.
[0036] Small-sized small-sized through holes 24 are formed in the
core substrate 12. The small-sized through holes 24 penetrate
through the core layer 13 and the insulating layers 22, 23. The
individual small-sized through hole 24 is located inside the
corresponding large-sized through hole 17. The small-sized through
hole 24 penetrates through the corresponding filling material 21.
Here, the small-sized through holes 24 each define a columnar
space. The individual small-sized through hole 24 is coaxial with
the corresponding large-sized through hole 17. The individual
small-sized through hole 24 defines circular openings on the front
and back surfaces of the core substrate 12, respectively.
[0037] A small-sized via 25 having a diameter smaller than that of
the large-sized via 18 is formed in the individual small-sized
through hole 24. The small-sized via 25 is electrically conductive.
The small-sized via 25 is formed in the shape of a cylinder along
the inward wall surface of the small-sized through hole 24. The
filling material 21 serves to insulate the large-sized via 18 and
the small-sized via 25 from each other. The small-sized via 25 is
made of an electrically-conductive material such as copper.
[0038] Electrically-conductive lands 26 are formed on the surfaces
of the insulating layers 22, 23. The small-sized via 25 is
connected to the electrically-conductive lands 26 on the surfaces
of the insulating layers 22, 23. The electrically-conductive lands
26 are made of an electrically-conductive material such as copper.
The inner space of the small-sized via 25 is filled with a filling
material 27 made of an insulating resin between the
electrically-conductive lands 26, 26. The filling material 27 is
formed in the shape of a column, for example. The filling material
27 is a thermosetting resin such as epoxy resin. Ceramic fillers
are embedded in the epoxy resin.
[0039] Electrically-conductive materials, namely vias 28, are
formed in the insulating layers 22, 23. The vias 28 are connected
to electrically-conductive patterns 29 formed on the back surfaces
of the insulating layers 22, 23, for example. Electrical connection
is in this manner established between the electrically-conductive
patterns 29 and the electrically-conductive layer 14 through the
vias 28, the electrically-conductive lands 19 and the small-sized
vias 25. The electrically-conductive layer 14 thus functions as the
power supply layer or the ground layer of the printed wiring board
11, for example. The vias 28 and the electrically-conductive
patterns 29 are made of an electrically-conductive material such as
copper.
[0040] Build-up layers 31, 32 are formed on the surfaces of the
insulating layers 22, 23, respectively. The back surfaces of the
build-up layers 31, 32 are received on the surfaces of the
insulating layers 22, 23, respectively. The core layer 13 and the
insulating layers 22, 23 are sandwiched between the build-up layers
31, 32. The build-up layers 31, 32 cover over the
electrically-conductive lands 26, 26 and the
electrically-conductive patterns 29.
[0041] The build-up layers 31, 32 are multilayered structure body
including insulating layers 33 and electrically-conductive patterns
34. The insulating layers 33 and the electrically-conductive
patterns 34 are alternatively overlaid on one another. Electrical
connection is established between the electrically-conductive
patterns 34 formed in different layers through vias 35. The
insulating layers 33 are made of a thermosetting resin such as
epoxy resin. The electrically-conductive patterns 34 and the vias
35 are made of an electrically-conductive material such as
copper.
[0042] Electrically-conductive pads 36 are exposed on the surfaces
of the build-up layers 31, 32. The electrically-conductive pads 36
are made of an electrically-conductive material such as copper. An
overcoat layer 37 is overlaid on the surface of each of the
build-up layers 31, 32 at positions off the electrically-conductive
pads 36. The overcoat layer 37 is made of epoxy resin, for example.
The electrically-conductive pads 36 on the back surface of the
printed wiring board 11 are connected to the electrode terminals on
a probe apparatus, for example. The electrically-conductive pads 36
on the front surface of the printed wiring board 11 are mounted on
the electrode bumps of a semiconductor wafer, for example. A heat
cycle test is then executed so as to examine the semiconductor
wafer, for example.
[0043] As shown in FIG. 2, all the large-sized through holes 17
located within a specific area are uniformly distributed. The
present embodiment allows a uniform distribution of all the
large-sized through holes 17 formed in the core substrate 12. Here,
four of the large-sized through holes 17 are positioned at the
corners of a square around an arbitrary one of the large-sized
through holes 17 located at the intersection of the diagonals for
achievement of a uniform distribution of the large-sized through
holes 17. The arbitrary one of the large-sized through holes 17 is
equally spaced from the large-sized through holes 17 at the corners
of the square.
[0044] In the printed wiring board 11, all the large-sized through
holes 17 located within a specific area have a specific identical
structure including the large-sized through hole 17, the
large-sized via 18, the filling material 21, the small-sized
through hole 24, the small-sized via 25 and the filling material
27. The filling material 21, 27 and the carbon fiber cloth are thus
uniformly distributed in the core substrate 12 in the in-plane
direction of the core substrate 12. This results in suppression of
uneven distribution of thermal stress in the core substrate 12 in
the in-plane direction of the core substrate 12. Cracks are
suppressed in the build-up layers 31, 32, for example. Break of the
electrically-conductive patterns 34 is avoided. Moreover, since all
the large-sized through holes 17 are uniformly distributed within
the specific area, suppression of uneven distribution of thermal
stress is promoted in the core substrate 12 in the in-plane
direction of the core substrate 12.
[0045] In the case where the printed wiring board has different
structures of the large-sized through holes in each area, the
carbon fiber cloth and the filling material are unevenly
distributed in the core substrate in the in-plane direction of the
core substrate. Since the coefficient of thermal expansion of the
carbon fiber cloth is different from that of the filling material,
thermal stress is unevenly distributed in the core substrate in the
in-plane direction of the core substrate during a heat cycle test,
for example. The uneven distribution causes cracks in the core
substrate. The cracks result in break of the
electrically-conductive pattern.
[0046] Next, description will be made on a method of making the
printed wiring board 11. The core substrate 12 is first prepared.
As shown in FIG. 3, four prepregs 41 are prepared, for example. The
individual prepreg 41 contains carbon fiber cloth. Simultaneously,
a pair of prepregs 42 is prepared, for example. The individual
prepreg 42 contains glass fiber cloth. The carbon fiber cloth is
impregnated with epoxy resin varnish for forming the prepregs 41.
Likewise, the glass fiber cloth is impregnated with epoxy resin
varnish for forming the prepregs 42. The epoxy resin varnish is
then dried. The prepregs 41, 42 are in this manner formed.
[0047] The prepregs 41 are sandwiched between the prepregs 42, 42.
The prepregs 42 are urged against each other while being subjected
to a heating process. A vacuum press is employed to urge the
prepregs 42, for example. The peak temperature of the heating
process and the pressure of the vacuum press are set in accordance
with predetermined conditions. The prepregs 41, 42 are bonded
together based on the molten epoxy resin. In this manner, the core
layer 13 is formed, as shown in FIG. 4. The prepregs 41 provides
form the electrically-conductive layer 14. The prepregs 42 form the
core insulating layers 15, 16, respectively.
[0048] As shown in FIG. 5, the through holes 17 are formed in the
core layer 13 at predetermined positions. A drill may be utilized
to form the through holes 17, for example. The through holes 17
penetrate through the core layer 13 from the front surface to the
back surface of the core layer 13. Electrolytic plating or
electroless plating is effected on the entire surface of the core
layer 13, for example. A copper plating layer 43 is thus formed
over the entire surface of the core layer 13, as shown in FIG. 6.
The copper plating layer 43 is formed along the front and back
surfaces of the core layer 13 and the inward wall surfaces of the
through holes 17 at a predetermined thickness. In this manner, the
large-sized vias 18 are formed inside the through holes 17.
[0049] As shown in FIG. 7, a resin material 44 is filled in the
large-sized vias 18. The resin material 44 is a solvent epoxy
resin, for example. The resin material 44 is subjected to a heating
process. The resin material 44 is thus hardened or cured. Buffing
is employed to remove the resin material 44 overflowing from the
large-sized vias 18, for example. A resist film, not shown, is then
formed on each of the front and back surfaces of the core layer 13
in a predetermined pattern. Etching is effected on the copper
plating layer 43 outside the contour of the resist film. The resist
film is removed after the etching. The electrically-conductive
lands 19 are thus formed on the front and back surfaces of the core
layer 13.
[0050] As shown in FIG. 8, a pair of prepregs 45, 45 are prepared.
The prepregs 45 have the same structure as that of the
aforementioned prepregs 42. The prepregs 45 are overlaid on the
front and back surfaces of the core layer 13, respectively. The
prepregs 45 are urged against the front and back surfaces of the
core layer 13 while being subjected to a heating process. A vacuum
press is employed to urge the prepregs 45. The peak temperature of
the heating process and the duration of the vacuum press are set in
accordance with predetermined conditions. The prepregs 45 are
bonded to the front and back surfaces of the core layer 13,
respectively, as shown in FIG. 9, based on the molten epoxy resin.
The prepregs 45 form the insulating layers 22, 23.
[0051] A UV-YAG laser is applied to the insulating layers 22, 23 at
predetermined positions, for example. Apertures 46 are thus formed
in the insulating layers 22, 23. The electrically-conductive lands
19 are exposed at the bottoms of the apertures 46, for example.
Electrolytic plating or electroless plating is effected on the
entire surface of the core layer 13, for example. A copper plating
layer 47 is thus formed over the entire surface of the core layer
13. The copper plating layer 47 is formed along the front and back
surfaces of the core layer 13 at a predetermined thickness. The
vias 28 are thus formed in the apertures 46.
[0052] As shown in FIG. 10, the small-sized through holes 24 are
formed in the resin material 44 inside the large-sized through
holes 17 of the core layer 13. The small-sized through holes 24 may
be coaxial with the corresponding large-sized through holes 17. A
drill may be utilized to form the small-sized through holes 24, for
example. Electrolytic plating or electroless plating is effected on
the entire surface of the core layer 13, for example. A copper
plating layer 48 is thus formed over the entire surface of the core
layer 13, as shown in FIG. 11. The copper plating layer 48 is
formed along the front and back surfaces of the core layer 13 and
the inward wall surfaces of the small-sized through holes 24 at a
predetermined thickness. The small-sized vias 25 are in this manner
formed in the small-sized through holes 24.
[0053] A resin material 49 is filled in the small-sized vias 25.
The resin material 49 is a solvent epoxy resin, for example. The
resin material 49 is subjected to a heating process. The resin
material 49 is thus hardened or cured. Buffing is employed so as to
remove the resin material 49 overflowing from the small-sized vias
25, for example. Electrolytic plating or electroless plating is
effected on the entire surface of the core layer 13, for example.
In this manner, a copper plating layer 51 is formed on each of the
front and back surfaces of the core layer 13, as shown in FIG. 12.
The openings of the small-sized through holes 24 are closed with
the copper plating layer 51. A resist film, not shown, is formed on
each of the front and back surfaces of the core layer 13 in a
predetermined pattern. Etching is effected on the copper plating
layer 51 outside the contour of the resist film. The
electrically-conductive lands 26 and the electrically-conductive
patterns 29 are thus formed on the front and back surfaces of the
core layer 13, as shown in FIG. 13. In this manner, the core
substrate 12 is formed.
[0054] Next, the build-up layers 31, 32 are formed on the front and
back surfaces of the core substrate 12, respectively. The build-up
layers 31, 32 are simultaneously formed. As shown in FIG. 14, a
resin sheet 52 is overlaid on each of the front and back surfaces
of the core substrate 12. The resin sheets 52 are urged against the
front and back surfaces of the core substrate 12 while being
subjected to a heating process. A vacuum press is executed to urge
the resin sheets 52. The peak temperature of the heating process
and the duration of the vacuum press are set in accordance with
predetermined conditions. The resin sheets 52 are hardened or cured
through the heating process. The resin sheets 52 thus provide the
insulating layers 33.
[0055] A UV-YAG laser is applied to the individual insulating layer
33 at predetermined positions, for example. Apertures 53 are thus
formed in the insulating layer 33, as shown in FIG. 15. The
electrically-conductive lands 26 are exposed at the bottom of the
apertures 53, for example. A copper plating layer 54 is formed on
the surface of the insulating layer 33 and inside the apertures 53.
Electroless plating is effected for forming the copper plating
layer 54, for example. A resist film, not shown, is formed on the
surface of the copper plating layer 53 in a predetermined pattern.
Etching is effected on the copper plating layer 54 outside the
contour of the resist film. The resist film is then removed. The
electrically-conductive pattern 34 is thus formed on the surface of
the insulating layer 33, as shown in FIG. 16. The vias 35 are
formed in the apertures 53.
[0056] The process of overlaying the insulating layer 33 and the
process of forming the electrically-conductive pattern 34 are
repeated. The aforementioned electrically-conductive pads 36 are
formed on the uppermost or exposed one of the insulating layers 33.
An overcoat layer, not shown, is formed on the surface of the
uppermost one of the insulating layers 33. The overcoat layer may
be made of the secondary filling material, for example. Screen
printing or photolithography may be effected for forming the
overcoat layer, for example. Openings are formed in the overcoat
layer at predetermined positions. The electrically-conductive pads
36 are exposed at the bottoms of the openings. The build-up layers
31, 32 are in this manner formed on the front and back surfaces of
the core substrate 12, respectively. The printed wiring board 11 is
produced.
[0057] FIG. 17 schematically illustrates the cross-section of a
printed wiring board 11a according to another example of the
present invention. The printed wiring board 11a includes the vias
28 connected to one ends of electrically-conductive patterns 61
extending along the front and back surfaces of the core layer 13.
As shown in FIG. 18, the other ends of the electrically-conductive
patterns 61 are connected to the electrically-conductive lands 19.
In this manner, electrical connection is established between the
electrically-conductive pattern 29 and the electrically-conductive
layer 14 through the large-sized vias 18, the
electrically-conductive lands 19, the electrically-conductive
pattern 61 and the vias 28. The electrically-conductive pattern 61
is made of an electrically-conductive material such as copper. Like
reference numerals are attached to the structure or components
equivalent to those of the aforementioned printed wiring board 11.
The printed wiring board 11a is allowed to enjoy the advantages
equivalent to those of the aforementioned printed wiring board
11.
[0058] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concept contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiments of the
present inventions have been described in detail, it should be
understood that the various changes, substitutions, and alterations
could be made hereto without departing from the spirit and scope of
the invention.
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